In order to attempt to meet a growing need of optical communication bandwidth, several methods for increasing the bandwidth of optical fibers have been implemented. Amongst these methods are: increasing the direct signal modulation speed, wavelength division multiplexing, coherent detection, polarization division multiplexing, and quadrature amplitude modulation. Further increase in the communication bandwidth is possible by utilization of a spatial degree of freedom, wherein different channels are differentiated spatially from each other. This method is known as mode division multiplexing (MDM).
The main advantage of MDM is its ability to incorporate all prior technologies, thus multiplying the total bandwidth by the number of available spatial channels. One of possible implementations of MDM is a multi-core fiber, in which several cores are present within a common cladding in a single fiber. Each core is used as a separate communication channel.
FIG. 1A and FIG. 1B schematically illustrate a prior art multicore fiber system. FIG. 1A shows the cross-sections of a single core fiber on the left and a seven core fiber on the right.
FIG. 1B shows a typical layout for a 109 Tbit transmission communication system 10 that is based on the use of a multi-core fiber. The input to the system is via seven individual single core fibers 12a, each of which is capable of transmitting 15.58 Tbits. The input fibers are connected by coupling device 14a to one end of a 16.8 km long seven core fiber 16. At the other end of the multi-core fiber a connector 14b connects each of the seven cores to an individual single core output fiber 12b. A drawback of this method is that it requires dedicated, complex, passive and active components such as filters, add drop components, amplifiers, switches, splitters, etc.
In order to reduce the complexity and cost it would be highly advantageous to utilize a single multimode fiber (MMF) core for transmitting various spatially separated communication channels. Amongst the names given to this method is mode division multiplexing (MDM). For example, one of the simplest, but not practical, ways to implement MDM is to use each mode as a separate channel.
Several implementation methods had been proposed and demonstrated for MDM. One of the methods utilizes a spatial light (phase) modulator (SLM). The laser light phase is intentionally altered in such a way, that it complies with particular selected mode launch conditions. In this way a spatially distinguishable communication channel is established. Several of these channels can be combined into a single fiber by a beam splitter element.
FIG. 2 schematically illustrates an example of a prior art fiber mode launch/detection system comprising a SLM. Light from a fiber source 18 is collimated by lens L1. The collimated light passes through a polarizing beam splitter (PBS) 20 and to a binary SLM 22. Light reflected from the SLM 22 is directed by the PBS 20 through lenses L2, L3, and L4 that focus it into the core at one end of MMF 24. The light exits at the other end of MMF 24 and passes through lens L5 and optical elements 26 that focus it on the focal plane of CCD 28.
An apparatus and method for increasing the communication bandwidth by reducing the modal dispersion in MMF using a SLM is taught by Kahn et al. in U.S. Pat. No. 7,844,144.
At the receiver end of the communication system, there is a need to spatially separate different modes from each other. In the optimal arrangement the modes would be focused on separate photodetectors; however, in most prior art schemes, a combination of spatial modes are detected by each photodetector and demultiplexing is applied to separate the modes using digital computations
One of the proposed methods to accomplish the mode separation is utilization of a volume hologram, which is especially designed to separate the desired modes from each other by using control light. FIG. 3 schematically shows a prior art volume hologram based method for separating the modes exiting a MMF at the optical receiver.
Both spatial light modulators and volume holograms are complicated free space optical devices. Therefore the possibilities of integrating them into photonic integrated circuits are very limited.
Since spatial mode separation at the receiver is complicated, digital multiplexing (MUX) and demultiplexing (DMUX) of the channels is used. This requires a-priori knowledge of the spatial mixing of channels and/or a dedicated feedback channel for enabling MUX and DMUX modules. Those modules are one of the main complexities of MDM.
FIG. 4 schematically shows a prior art layout of a mode division multiplexing based optical communication link. In this method N input signals 30 are processed by signal processing unit 32, which outputs N sources 34. The light from sources 34 enters one end of multimode fiber 36 and exits at the other end where it is detected by N detectors 38. The detected signals are sent from detectors 38 to signal processing unit 40, which outputs N recovered signals 42. To accomplish mode separation at the receiver end a feedback signal 44 is created by signal processing unit 40. Feedback signal 44 activates laser 46, whose output travels through MMF 36 and is detected by detector 48. The output signal 50 from detector 48 is feed into signal processing unit 32 and utilized in generating sources 34.
Another method for launching spatially distinguishable modes into a gradient index multimode fiber (GI-MMF) is focusing of a single mode beam onto a GI-MMF core with spatial offset from center. In this way spatially distinguishable modes are obtained and a known DMUX matrix can be used to distill the data from each channel. One of the main advantages of GI-MMF is its reduced mode mixing properties, while it maintains its modal structure for 1-2 km. This is important since, on the one hand, mode division multiplexing requires maximum separation between the modes or group of modes and, on the other hand, multi-mode fibers exhibit mode mixing when the fiber bends and stress and imperfections cause different modes to exchange power between them.
Another demonstration of MDM was done by a selective launch of few first modes into a few mode fiber (FMF) and subsequent use of phase plates and beam splitters for modes separation at the receiver end.
Each of the methods described above has its drawbacks that have prevented it from being universally accepted and used in practical optical communication systems.
It is therefore a purpose of the present invention to provide a method for mode division multiplexing in an optical fiber communication system that is relatively simple and economical to carry out.
Further purposes and advantages of this invention will appear as the description proceeds.